Recombinant protein production constitutes an important application of transgenic plants. In addition to the yield and the favorable cost of the field production of recombinant proteins, transgenic plants present certain advantages over other production systems, such as bacteria, yeasts, and animal cells. Indeed, they are devoid of agents infectious to humans, and can accumulate the proteins of interest in storage tissues, such as seeds or tubers. This facilitates their handling, their transportation and their storage at ambient temperature, while affording the possibility of subsequent extraction. Moreover, the transgenic plant, or some of its parts, can be utilized as vector of medicaments or of vaccines.
Although the advantages of plants as factories of proteinaceous substances are explained mostly in the light of biopharmaceuticals, plants are also useful for production of other proteins, e.g., industrial enzymes and the like, because of their capability of glycosylation leading e. g. to higher stability. Today, the utilization of plants for the production of heterologous glycoproteins for therapeutic and other use is investigated in soy, tobacco, potato, rice and rapeseed, and the glycoproteins produced therein include monoclonal antibodies, hormones, vaccine antigens, enzymes and blood proteins. Some of these proteins have already proven their efficacy in humans.
A drawback of glycoprotein production in plants relates to the glycosylation pattern of the glycoproteins produced in plants. Like other heterologous expression systems, plants exhibit a different glycosylation profile compared to mammals. In contrast to bacteria, having no N-linked glycans, and yeast, having only N-linked glycans of the high mannose type, plants are able to produce proteins with N-linked glycans of the complex type. However, plant glycoproteins have complex N-linked glycans containing β1,2-xylose and α1,3-fucose residues not found in mammals. Moreover, plant glycoproteins lack the characteristic galactose-containing complex N-glycans found in mammals.
In short, analyses of glycoproteins from plants have indicated that, although similarities exist, several steps in the glycosylation pathways of plants and mammals are different, particularly in the synthesis of complex glycans. The complex glycans of plants are generally much smaller and contain beta-1,2 xylose or alpha-1,3 fucose residues attached to the Man3(GlcNAc)2 core. Such residues on glycoprotein are known to be immunogenic, which causes problems for certain applications of recombinant proteins carrying these sugars.